The laser evaporation method described above is used widely particularly on the research level in order to form thin films of many different materials including: oxide ferroelectric substances, insulators, conducting oxides, oxide superconductors, oxide magnetic materials, etc. This is because it has many advantages and is very promising.
FIG. 7 illustrates an example of the basic constitution of the apparatus used in this method, so to first explain the basic points thereof, a laser beam LB is shined from outside through a laser beam introduction window onto one or more targets 12 placed within a deposition chamber 13, the interior of which is evacuated to a stipulated degree of vacuum by means of a vacuum exhauster 14. Thereupon, the material making up the target in the vicinity of the surface of the target upon which the laser beam LB shines evaporates (ablates), and this target material is dispersed in the form of a type of mist-like mass called a plume 15 which is the light-excited state of the evaporated material that extends in a direction normal to the target and expands roughly symmetrically centered about this normal line, and this is deposited upon a substrate 11 supported by a substrate holder 21. A gas Gs may be introduced within the deposition chamber 13 if necessary; for example, oxygen is often introduced to form oxides, or nitrogen to form nitrides. In addition, the substrate 11 is typically heated by a heater 16 provided on the substrate holder 21, again if necessary.
With such a laser evaporation method, only those portions of the targets 12 upon which light is incident are locally ablated, so it is desirable that there is a tendency for the compound state of the target material be reflected straight into the state of the compound deposited upon the substrate. In addition, there is an advantage in that the optimal value for the pressure of oxygen or other gas introduced within the vessel can be very readily selected in a wide range from low vacuum to a high vacuum state. A film forming method that is often compared with this technique is the sputtering method, but this requires a discharge to be induced within the vessel and the main gas therein is argon. Accordingly, even if oxygen is introduced, the variable range of pressures is narrow, and if the object is to form an oxide film, the proper conditions often cannot be selected. In fact, an oxide superconductor, for example, produced by the laser evaporation method has a critical transition temperature Tc equivalent to the bulk physical properties, and in the case of forming a thin film of a ferroelectric substance, its polarity value is large and it is structurally dense with a small leak current, so superior thin-film characteristics can often be obtained.
In this manner, the laser evaporation method has many superior advantages such as the superiority of the physical property control and characteristics of the formed thin film, the good versatility in that a considerable number of types of thin films can be produced, and the like, but there were still problems that remained to be solved. These were problems with the uniformity of thickness of the formed thin film, in that the distribution of thicknesses of thin films formed upon the substrate has up until now had a considerable range of variation. This was because, as described above, the plume 15 is formed from the spot where the laser shines onto the target (the target spot) in a direction nearly normal to the target, so the amount of deposition is greatest at the portion of the substrate corresponding to areas along the centerline of the plume 15, with deposition decreasing rapidly away from the centerline. This drawback naturally becomes more marked the greater the substrate surface area becomes, so the current situation is such that it is completely unsuited to film formation over large surface areas.
In fact, in order to solve this drawback, in the past, various improvements have been proposed such as moving or rotating the substrate, scanning the laser beam relative to the substrate and the like. However, as representative prior-art documents, even though Document 1: JP-Hei-5-255842A and Document 2: JP-Hei-11-246965A and others have demonstrated improvements, satisfactory results have still not been obtained, and although the range of fluctuation in the film thickness distribution was 10% or less in the former document, it has been difficult to reduce this to a range of 1-2%, and this was only roughly 6% even in the latter document, so a uniform film thickness has not been obtained.
Naturally, achieving a uniform film thickness distribution is a problem that is to be solved not only in the PLD method but also in the sputtering method described above and other film forming methods.